Dispatch communication systems using transmission trunking are known. Such systems typically use frequency pairs (i.e., a transmit frequency and a receive frequency) that are assigned by a resource controller for transmitting and receiving messages. The resource controller assigns the frequency pair upon receiving a request for service from a communications unit. Upon receiving the assignment data the requesting unit and all target units tune to the appropriate frequencies and two-way communication can then occur.
While most dispatch communications systems utilize a single site, some utilize multiple sites, or cells to cover a larger geographic area. As is known in the art, if less than all of the target communication units are located in the same cell as the requestor then the system controller must identify the locales of the remaining target units. The locales are normally identified thorough the use of one of two known methods: system wide paging, or cell registration.
In system wide paging the locale of a target unit is determined by transmitting a page to the target unit over a resource controller in every cell within the system. The locale of the target is then determined from a response by the target to the resource controller in the cell within which the target unit is located.
In cell registration a communication unit transmits its identity to the resource controller within a cell whenever the communication unit passes into the coverage area of the cell, as in the case where a communication unit moves through a geographic area covered by multiple cells.
In the past, dispatch communication systems have worked well because systems tended to have few (or only one) large cells and competition for spectral space was minimal. However, cells have decreased in size through the passage of time (increased in number) as a result of increased competition for spectral resources. As such, system wide paging is often not appropriate for all users as it is wasteful of spectral resources in systems with many cells as in the case of communications users that use the system frequently, i.e., that utilize the communications systems much more often than they change cell locations. Also, the system wide page increases the time required to access targets, as the paging process requires significant time to locate target units.
Likewise, systems which employ cell registration may also be wasteful of spectral resources as in the case where the communication unit changes cell location more frequently than the communication unit utilizes the communications system or when a communication unit is primarily a call initiator and seldom is a call target. Accordingly, a need exists for a methodology for finding target communication units that is less wasteful of time and spectrum.